Knives and Pens is about coping by doing something creative or writing instead of harming yourself. Many people turn to self-harming after being bullied and for other reasons. This song encourages people to find another way to cope, instead of cutting / self-harming.
Basically, you choose to either harm yourself (Knives/blade/etc.) or find a better and more positive way to release and relieve stress (pens/writing/music/singing/etc.)
-By the way, if you watched the video, you can see there is a kid being bullied.